Investments - Schedule of Carrying Value and Unfunded Investment Commitments of Other Invested Assets Portfolio (Detail) - USD ($) $ in Millions	Sep. 30, 2019	Dec. 31, 2018
Schedule of Investments [Line Items]
Carrying Value	$ 5,127.0	$ 4,787.0	[1]
Hedge funds [Member]
Schedule of Investments [Line Items]
Carrying Value	119.3	120.6
Unfunded Commitments	0.0	0.0
Private equity [Member]
Schedule of Investments [Line Items]
Carrying Value	263.9	211.8
Unfunded Commitments	94.2	120.5
Long only funds [Member]
Schedule of Investments [Line Items]
Carrying Value	112.5	153.0
Unfunded Commitments	0.0	0.0
Other [Member]
Schedule of Investments [Line Items]
Carrying Value	4.3	4.4
Unfunded Commitments	0.0	0.0
Other Investments [Member]
Schedule of Investments [Line Items]
Carrying Value	500.0	489.8
Unfunded Commitments	$ 94.2	$ 120.5

[1] Derived from audited consolidated financial statements.